The primary focus in evaluating a large concrete structure, such as a dam, is to determine the ability of the structure to withstand natural disasters such as a major earthquake or serious flooding. The reaction of the structure depends on the mechanical strength developed throughout the complex discontinuous mass of the structure. Presently used construction techniques, combined with normal loading conditions, ensure that most defects in a large concrete dam structure will develop in horizontal planes along construction joints, also known as lift-lines. The survival of a dam under extreme loading conditions often depends on the strength developed across these construction joints or lift-lines.
The traditional method for determining the strength of a concrete dam is to extract large diameter core samples drilled from the top of the dam down to the foundation, which samples are then destructively tested to determine the strength and elastic modulus thereof. These material data are then used in a finite element model to simulate responses of the dam structure to various loading conditions. The parameters required for finite element models include the modulus of elasticity, strength, density, and Poisson's ratio. The location and extent of any defects within the structure are also required. Depending on the size of the structure, two to four drill holes are cored down to the foundation, often a distance of more than 90 meters (300 feet). Large diameters cores, 250-300 millimeters (10 to 12 inches), are required to provide representative samples. It will be appreciated that extracting this amount of large diameter concrete cores is very expensive. In addition, although a coring program of this magnitude is considered sufficient, it only samples a very small percentage of the volume of the dam's structure, typically less than 0.1%. This procedure cannot, therefore, be expected to locate most local anomalies, such as regions of disbonded lift-lines or cracks. A new testing procedure is required which will provide a more thorough evaluation of the physical properties of the structure, and preferably one which is less expensive than a full coring program.